Set Screws: From Door Knobs to Space Shuttles, What Are They and When Should You Use Them?

Set screws are one of those smaller metric fastener components that aren’t given as much attention as they deserve.

In spite of their lack of fame, they are available in a wide variety of different fabrications. They are also used in a variety of different applications ranging from simple tasks such as installing a door knob, to more complex ones like working on a space shuttle.

Any time an object needs to be affixed to some sort of shaft, a set screw is likely to be utilized. However, given the wide variety of applications that use set screws, you want to make sure that you are using the right one for your specific application.

What, Exactly, is a Set Screw?
Simply put, a set screw is a threaded fastener that creates tension between two surfaces.

One of the easiest ways to picture this is with a gear and shaft assembly. In this setup, a gear is seated onto a shaft. Torsion is applied to the shaft, causing the gear to turn, or vice versa. Without something to create tension between the outer surface of the shaft and the inner surface of the gear, the two would simply spin independently of one another.

To solve this problem, a threaded hole is machined in the collar surrounding the hole in the center of the gear. This hole extends through the collar to the surface of the shaft. A set screw is then inserted into the threaded hole and tightened against the shaft. This creates tension that forces the gear and shaft to turn in unison. In some applications, a flat spot, or detente, is machined into the surface of the shaft to increase tension.

The Right Set Screw for the Right Application
There are, of course, many variations on this simple setup. As such, there are a variety of set screws to meet the needs of any assembly. Knowing which screw to use for each application is important for the success of the assembly.

Flat Set ScrewsFlat set screws have a drive end for the installation tool, and a flattened end to create tension with the mating surface. These set screws are frequently used with a detente to increase holding power. They are ideal for applications where a non-marring connection is desired. Flat set screws allow for easy adjustment of the screw location, which is ideal for applications where frequent resetting and relocation of connected components is necessary. Once moved, there is little sign of the previous point of contact with the set screw.

Cone Set ScrewsCone set screws are intended for more permanent installations. They have a hardened point on the mating end, which penetrates the surface of the shaft upon tightening. This provides greater tension against the shaft, allowing for higher loads without slipping. Since the cone set screws penetrate the surface of the shaft, there is no need for milling a detente in the shaft. This can reduce milling costs, and prevent balancing issues in high tolerance fabrications.

Dog Set ScrewA dog set screw is used with a detente or with a hole drilled in the surface of the shaft. These set screws have a protruding tip that rests against the detente, or fits into a pre-drilled hole in the shaft. They are intended for permanent installations, and provide superior torsional and axial holding. Once installed correctly, dog set screws will not slip unless overloaded to the point of shearing.

Hold on Tight
Due to their reliability, adjustability, and ease of use, set screws are still preferred over other methods of attachment. When it comes to affixing components to a shaft, set screws are favored over welding, dowels, pins, and other types of attachment.

About the author

Mr. Metric is the spokesperson for www.mrmetric.com, an online store for metric fasteners. Get $7.50 flat shipping with no minimum orders, plus advice and best practices for using metric fasteners in any application.